Okay, let's break down series ratings and apply them to your scenario, based on the image you linked.

Understanding Series Ratings What it is

Series rating (also called a "series combination rating") is a UL-approved, tested, and listed way of using a higher-rated upstream (feeder) breaker to protect a lower-rated downstream (branch) breaker in the event of a fault. It relies on the fact that the upstream breaker will limit the current seen by the downstream breaker during a short circuit.

Why use it

It can save money by allowing you to use less expensive, lower-rated breakers downstream, rather than having to use fully rated breakers throughout the entire system.

Important Considerations



Manufacturer's Listing is KEY

You absolutely MUST follow the exact series rating combinations as specified by the breaker manufacturer's listing. This information is generally available in their product documentation. You

cannot arbitrarily combine breakers and assume they're series rated. If it's not listed, it's not series rated. Specific Breakers/Models

Series ratings are for specific breaker models, ampacities, and manufacturers. You can't just say "any breaker of Brand X is series rated with any breaker of Brand Y." It's much more granular than that.

Maximum Fault Current

The available fault current at the downstream breaker location must be within the series rating that the manufacturer lists for the combination.

Selective Coordination is NOT Guaranteed

Series ratings are primarily for protection against short circuits. They do NOT guarantee selective coordination. A downstream fault might still trip the upstream breaker, even if the series rating is met.

UL Listing

The overall assembly must be UL listed for the series rating to be valid.

Analyzing Your Scenario

Let's make some assumptions for clarity, because a lot depends on the specifics of the equipment. Also I will assume this is a standard electrical installation in the USA and 480/277V. If this is incorrect, please advise.

Assumptions



Feeder Breaker

The breaker supplying Pnl1 is what would need to have testing data to protect the downstream breakers

Pnl1, Pnl2, Pnl3

These are distribution panels containing multiple branch circuit breakers.

All Breakers are from Reputable Manufacturers

Brands like Eaton, Siemens, Schneider Electric, GE, etc.

We can find the Manufacturer's Series Rating Tables

This is essential.

1) Can Pnl1's Lowest Rated Breaker be Series Rated to the Feeder Breaker?Potentially, YES, IF: The manufacturer of the main/feeder breaker AND the breakers in Pnl1 has published a series rating combination that includes the specific breaker models, ampacities, and short-circuit current rating for both the upstream (feeder) breaker and the downstream breakers in Pnl1. The available fault current at Pnl1 is less than or equal to the series rating listed by the manufacturer for that breaker combination. This requires a fault current study. You are using the exact breakers specified in the series rating listing. This is critical. Both breakers are standard molded case breakers (MCCB). A common series rating is 100kAIC.Example

Let's say you have an Eaton series-rated combination where an Eaton Series G HMCPS main breaker is series rated to protect Eaton Series C branch breakers in the Pnl1 panel. The manufacturer's documentation might say:_"Eaton Series G HMCPS 200A breaker (100 kAIC) can be series rated with Eaton Series C 20A, 30A, and 50A breakers (14 kAIC) up to an available fault current of 65 kA at the downstream breaker location."_In this example: You MUST use the Eaton Series G HMCPS 200A breaker upstream. You MUST use Eaton Series C 20A, 30A or 50A breakers downstream. The available fault current at the Pnl1 location MUST NOT exceed 65 kA.

2) Can the Series Rating Continue Down to Pnl3?Potentially, YES, BUT it's more complex and less likely in practice:This is where it gets tricky. A single feeder breaker series-rating down to multiple panelboards would likely not be a valid situation. To be series rated, it would have to be a tested situation for each panelboard with a published series rating list for each panelboard. The manufacturer would have had to test and publish this specific series rating "chain" or "cascade" of breakers. You would need to find documentation that specifically allows a series rating from the feeder breaker through Pnl1 down to Pnl3. The available fault current at both Pnl1 and Pnl3 would need to be less than or equal to the series rating. You would need to use the exact breakers specified in the listing at each level.In MOST cases, series ratings are done on a SINGLE breaker to SINGLE downstream breaker basis. Chaining them like this is much less common, and you may not find a tested combination.

Alternatives

If you are unable to achieve this situation with listed series rating data, you will need to review the fault current study to confirm that each panel has adequate AIC rating of the breakers installed.

Crucial Steps1. Fault Current Study

Get a detailed fault current study done for your electrical system. This is essential to determine the available fault current at each panelboard (Pnl1, Pnl2, and Pnl3).2.

Manufacturer Documentation

Contact the breaker manufacturers (for both the main breaker AND the breakers in your panelboards) and request their series rating tables or documentation. Tell them exactly what breaker combinations you are trying to achieve.3.

Comparison

Compare the fault current study results to the manufacturer's series rating tables. If the fault current is within the allowable limits, and you are using the correct breaker combinations, then you can apply the series rating.4.

Documentation

Keep meticulous records of the manufacturer's series rating information, the fault current study, and the breaker specifications for each panelboard.5.

Inspection

Ensure the installation and labeling are correct and matches the approved drawings and documentation to pass inspection.

In summary: Series ratings are highly specific. Manufacturer's documentation is the key. Fault current studies are essential. Chaining series ratings down to multiple panelboards is less common.Good luck! Always consult with a qualified electrical engineer or professional engineer to ensure your electrical system is safe and code-compliant. Flag for review